Sensors Measurement is an important subsystem of a

Sensors

• Measurement is an important subsystem of a mechatronics system. Its main function is to collect the information on system status and to feed it to the micro-processor(s) for controlling the whole system. • Measurement system comprises of sensors, transducers and signal processing devices.

Role of Sensors for Mechatronics 1. Sensors alarm the system operators about the failure of any of the sub units of manufacturing system. It helps operators to reduce the downtime of complete manufacturing system by carrying out the preventative measures. 2. Reduces requirement of skilled and experienced labors. 3. Ultra-precision in product quality can be achieved.

Sensor/transducers specifications • Transducers or measurement systems are not perfect systems. Mechatronics design engineer must know the capability and shortcoming of a transducer or measurement system to properly assess its performance.

Definition of Sensor • Sensor is an element which produces signal relating to the quantity being measured. • Sensor can be defined as “A device which provides a usable output in response to a specified measurand. ” • The output is usually an ‘electrical quantity’ and measurand is a ‘physical quantity, property or condition which is to be measured’. • For example, in the case of, say, a variable inductance displacement element, the quantity being measured is displacement and the sensor transforms an input of displacement into a change in inductance.

Transducers • It is defined as an element when subjected to some physical change experiences a related change [1] or an element which converts a specified measurand into a usable output by using a transduction principle. • It can also be defined as a device that converts a signal from one form of energy to another form. • ‘sensors are transducers’.

1. Range • The range of a sensor indicates the limits between which the input can vary. For example, a thermocouple for the measurement of temperature might have a range of 25 -225 °C. 2. Span The span is difference between the maximum and minimum values of the input. Thus, the abovementioned thermocouple will have a span of 200 °C.

3. Error • Error is the difference between the result of the measurement and the true value of the quantity being measured. A sensor might give a displacement reading of 29. 8 mm, when the actual displacement had been 30 mm, then the error is – 0. 2 mm. 4. Accuracy • The accuracy defines the closeness of the agreement between the actual measurement result and a true value of the measurand. It is often expressed as a percentage of the full range output or full–scale deflection. A piezoelectric transducer used to evaluate dynamic pressure phenomena associated with explosions, pulsations, or dynamic pressure conditions in motors, rocket engines, compressors, and other pressurized devices is capable to detect pressures between 0. 1 and 10, 000 psig (0. 7 KPa to 70 MPa). If it is specified with the accuracy of about ± 1% full scale, then the reading given can be expected to be within ± 0. 7 MPa.

5. Sensitivity • Sensitivity of a sensor is defined as the ratio of change in output value of a sensor to the per unit change in input value that causes the output change. For example, a general purpose thermocouple may have a sensitivity of 41 μV/°C.

6. Nonlinearity • The nonlinearity indicates the maximum deviation of the actual measured curve of a sensor from the ideal curve. Figure 2. 1. 1 shows a somewhat exaggerated relationship between the ideal, or least squares fit, line and the actual measured or calibration line. Linearity is often specified in terms of percentage of nonlinearity which is defined as: Nonlinearity (%) = Maximum deviation in inpu ⁄ Maximum full scale input (2. 1. 1) The static nonlinearity defined by Equation 2. 1. 1 is dependent upon environmental factors, including temperature, vibration, acoustic noise level, and humidity. Therefore it is important to know under what conditions the specification is valid.

7. Hysteresis The hysteresis is an error of a sensor, which is defined as the maximum difference in output at any measurement value within the sensor’s specified range when approaching the point first with increasing and then with decreasing the input parameter. Figure 2. 1. 2 shows the hysteresis error might have occurred during measurement of temperature using a thermocouple. The hysteresis error value is normally specified as a positive or negative percentage of the specified input range.

8. Resolution • Resolution is the smallest detectable incremental change of input parameter that can be detected in the output signal. Resolution can be expressed either as a proportion of the full-scale reading or in absolute terms. For example, if a LVDT sensor measures a displacement up to 20 mm and it provides an output as a number between 1 and 100 then the resolution of the sensor device is 0. 2 mm. 9. Stability • Stability is the ability of a sensor device to give same output when used to measure a constant input over a period of time. The term ‘drift’ is used to indicate the change in output that occurs over a period of time. It is expressed as the percentage of full range output.

10. Dead band/time • The dead band or dead space of a transducer is the range of input values for which there is no output. The dead time of a sensor device is the time duration from the application of an input until the output begins to respond or change. 11. Repeatability • It specifies the ability of a sensor to give same output for repeated applications of same input value. It is usually expressed as a percentage of the full range output: • Repeatability = (maximum – minimum values given) X 100 ⁄ full range

12. Response time • Response time describes the speed of change in the output on a step-wise change of the measurand. It is always specified with an indication of input step and the output range for which the response time is defined.

Classification of sensors A. Displacement, position and proximity sensors • Potentiometer • Strain-gauged element • Capacitive element • Differential transformers • Eddy current proximity sensors • Inductive proximity switch • Optical encoders • Pneumatic sensors • Proximity switches (magnetic) • Hall effect sensors

B. Velocity and motion • Incremental encoder • Tachogenerator • Pyroelectric sensors C. Force • Strain gauge load cell D. Fluid pressure • Diaphragm pressure gauge • Capsules, bellows, pressure tubes • Piezoelectric sensors • Tactile sensor

E. Liquid flow • Orifice plate • Turbine meter F. Liquid level • Floats • Differential pressure G. Temperature • Bimetallic strips • Resistance temperature detectors • Thermistors • Thermo-diodes and transistors • Thermocouples • Light sensors • Photo diodes • Photo resistors • Photo transistor

Displacement and position sensors • Displacement sensors are basically used for the measurement of movement of an object. Position sensors are employed to determine the position of an object in relation to some reference point. Figure shows the construction of a Schematic of a potentiometer sensor for measurement of linear displacement rotary type potentiometer sensor employed to measure the linear displacement. The potentiometer can be of linear or angular type. It works on the principle of conversion of mechanical displacement into an electrical signal. The sensor has a resistive element and a sliding contact (wiper). The slider moves along this conductive body, acting as a movable electric contact.

Sensor in Control System

• Above Figure shows the construction of a rotary type potentiometer sensor employed to measure the linear displacement. The potentiometer can be of linear or angular type. • It works on the principle of conversion of mechanical displacement into an electrical signal. • The sensor has a resistive element and a sliding contact (wiper). The slider moves along this conductive body, acting as a movable electric contact.

• Applications of potentiometer • These sensors are primarily used in the control systems with a feedback loop to ensure that the moving member or component reaches its commanded position. • These are typically used on machine-tool controls, elevators, liquid-level assemblies, forklift trucks, automobile throttle controls. In manufacturing, these are used in control of injection molding machines, woodworking machinery, printing, spraying, robotics, etc. These are also used in computer-controlled monitoring of sports equipment.

2. Strain Gauges • The strain in an element is a ratio of change in length in the direction of applied load to the original length of an element. The strain changes the resistance R of the element. Therefore, we can say, ΔR/R α ε; ΔR/R = G ε where G is the constant of proportionality and is called as gauge factor. Resistance strain gauge follows the principle of change in resistance as per the above equation.

The change in resistance of the strain guage can be detected by a using a Wheatstone’s resistance bridge. In the balanced bridge we can have a relation, R 2/ R 1 = Rx / R 3 where Rx is resistance of strain gauge element, R 2 is balancing/adjustable resistor, R 1 and R 3 are known constant value resistors. R 2 is changed so that Vo to be zero. After that, we can compute value of Rx by measuring the value of R 2, R 1/(R 1+R 2)=R 3/(R 3+Rx) R 1(R 3+Rx)=R 3(R 1+R 2) R 2/R 1=Rx/R 3

Applications of strain gauges • Strain gauges are widely used in experimental stress analysis and diagnosis on machines and failure analysis. They are basically used for multi-axial stress fatigue testing, proof testing, residual stress and vibration measurement, torque measurement, bending and deflection measurement, compression and tension measurement and strain measurement. • Strain gauges are primarily used as sensors for machine tools and safety in automotives. In particular, they are employed force measurement in machine tools, hydraulic or pneumatic press and as impact sensors in aerospace vehicles.

Capacitive element based sensor • Capacitive sensor is of non-contact type sensor and is primarily used to measure the linear displacements from few millimeters to hundreds of millimeters. It comprises of three plates, with the upper pair forming one capacitor and the lower pair another. The linear displacement might take in two forms: a. one of the plates is moved by the displacement so that the plate separation changes b. area of overlap changes due to the displacement.

The capacitance C of a parallel plate capacitor is given by, C = εr εo A / d Displacement measurement using capacitive element sensor

Applications of capacitive element sensors • Feed hopper (곡물을 아래로 보내는 용기)level monitoring • Small vessel pump control • Grease level monitoring • Level control of liquids • Metrology applications o to measure shape errors in the part being produced o to analyze and optimize the rotation of spindles in various machine tools such as surface grinders, lathes, milling machines, and air bearing spindles by measuring errors in the machine tools themselves • Assembly line testing o to test assembled parts for uniformity, thickness or other design features o to detect the presence or absence of a certain component, such as glue etc.

4. Linear variable differential transformer (LVDT) • Linear variable differential transformer (LVDT) is a primary transducer used for measurement of linear displacement with an input range of about ± 2 to ± 400 mm in general. It has non-linearity error ± 0. 25% of full range. The following figure shows the construction of a LVDT sensor. It has three coils symmetrically spaced along an insulated tube. • The central coil is primary coil and the other two are secondary coils. Secondary coils are connected in series in such a way that their outputs oppose each other. • A magnetic core attached to the element of which displacement is to be monitored is placed inside the insulated tube.

Working of LVDT sensor • When the magnetic core is centrally placed with its half portion in each of the secondary coil regions then the resultant voltage is zero. • LVDT exhibits good repeatability and reproducibility. It is generally used as an absolute position sensor. Since there is no contact or sliding between the constituent elements of the sensor, it is highly reliable. These sensors are completely sealed and are widely used in Servomechanisms, automated measurement in machine tools. • A rotary variable differential transformer (RVDT) can be used for the measurement of rotation.

Applications of LVDT sensors • Measurement of spool (실 감는 패) position in a wide range of servo valve applications • To provide displacement feedback for hydraulic cylinders • To control weight and thickness of medicinal products viz. tablets or pills • For automatic inspection of final dimensions of products being packed for dispatch • To measure distance between the approaching metals during Friction welding process • To continuously monitor fluid level as part of leak detection system • To detect the number of currency bills dispensed by an ATM

Displacement, position and proximity sensors • Eddy current proximity sensors are used to detect non-magnetic but conductive materials. They comprise of a coil, an oscillator, a detector and a triggering circuit.

• When an alternating current is passed thru this coil, an alternative magnetic field is generated. If a metal object comes in the close proximity of the coil, then eddy currents are induced in the object due to the magnetic field. These eddy currents create their own magnetic field which distorts the magnetic field responsible for their generation. As a result, impedance of the coil changes and so the amplitude of alternating current. This can be used to trigger a switch at some pre-determined level of change in current.

• Eddy current sensors are relatively inexpensive, available in small in size, highly reliable and have high sensitivity for small displacements. Applications of eddy current proximity sensors • Automation requiring precise location • Machine tool monitoring • Final assembly of precision equipment such as disk drives • Measuring the dynamics of a continuously moving target, such as a vibrating element, • Drive shaft monitoring • Vibration measurements

2. Inductive proximity switch The inductance of the loop changes according to the material inside it and since metals are much more effective inductors than other materials the presence of metal increases the current flowing through the loop. This change can be detected by sensing circuitry, which can signal to some other device whenever metal is detected. • Inductive proximity switches are basically used for detection of metallic objects. • Above Figure shows the construction of inductive proximity switch. An inductive proximity sensor has four components; the coil, oscillator, detection circuit and output circuit. • An alternating current is supplied to the coil which generates a magnetic field. When, a metal object comes closer to the end of the coil, inductance of the coil changes. This is continuously monitored by a circuit which triggers a switch when a preset value of inductance change is occurred.

Applications of inductive proximity switches • Industrial automation: counting of products during production or transfer • Security: detection of metal objects, arms, land mines

3. Optical encoders Construction and working of optical encoder

• Optical encoders provide digital output as a result of linear / angular displacement. These are widely used in the Servo motors to measure the rotation of shafts. • It comprises of a disc with three concentric tracks of equally spaced holes. Three light sensors are employed to detect the light passing thru the holes. These sensors produce electric pulses which give the angular displacement of the mechanical element e. g. shaft on which the Optical encoder is mounted. • The resolution can be determined by the number of holes on disc. With 100 holes in one revolution, the resolution would be, 360⁰/100 = 3. 6⁰. •

4. Pneumatic Sensors Working of Pneumatic Sensors Pneumatic sensors are used to measure the displacement as well as to sense the proximity of an object close to it. The displacement and proximity are transformed into change in air pressure.

• It comprises of three ports. Low pressure air is allowed to escape through port A. In the absence of any obstacle / object, this low pressure air escapes and in doing so, reduces the pressure in the port B. • However when an object obstructs the low pressure air (Port A), there is rise in pressure in output port B. This rise in pressure is calibrated to measure the displacement or to trigger a switch. • These sensors are used in robotics, pneumatics and for tooling in CNC machine tools.

5. Proximity Switches Configurations of contact type proximity switch These are small electrical switches which require physical contact and a small operating force to close the contacts. They are basically employed on conveyor systems to detect the presence of an item on the conveyor belt.

Magnet based Reed switches are used as proximity switches. When a magnet attached to an object brought close to the switch, the magnetic reeds attract to each other and close the switch contacts.

Photo emitting devices such as Light emitting diodes (LEDs) and photosensitive devices such as photo diodes and photo transistors are used in combination to work as proximity sensing devices. This figure shows two typical arrangements of LEDs and photo diodes to detect the objects breaking the beam and reflecting light. LED based proximity sensors

6. Hall effect sensor Principle of working of Hall effect sensor

• Hall effect sensors work on the principle that when a beam of charge particles passes through a magnetic field, forces act on the particles and the current beam is deflected from its straight line path. Thus one side of the disc will become negatively charged and the other side will be of positive charge. This charge separation generates a potential difference which is the measure of distance of magnetic field from the disc carrying current.

• The typical application of Hall effect sensor is the measurement of fluid level in a container. The container comprises of a float with a permanent magnet attached at its top. An electric circuit with a current carrying disc is mounted in the top casing. When the fluid level increases, the magnet will come close to the disc and a potential difference generates. This voltage triggers a switch to stop the fluid to come inside the container. • Hall effect sensors need necessary signal conditioning circuitry. They can be operated at 100 k. Hz. Their noncontact nature of operation, good immunity to environment contaminants and ability to sustain in severe conditions make them quite popular in industrial automation.